Recreate gravel bar and riffles

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Recreate gravel bar and riffles

Recreate gravel bar and riffles06. In-channel structure and substrate improvement

General description

In few words: Install structures to encourage sediment accretion and localized diversity in channel bedforms.

Gravel bar recreation

Natural erosion and sedimentation processes in stream bed and margins originate the creation of lateral bars with alluvial materials. The bar recreation consists in simulate these natural structures which are developed in streams with medium or strong solid sediment transport. Normally the migration of coarse sediment takes place with the characteristic form of alternate bars. These structures promote lateral erosion and initiate the development of sinuosity or meandering.

Depending on the geodynamic characteristics and the solid contributions of the stream the principles for applying this technique will vary.

• Medium to strong solid transportation. In this case simply by creating structures blocking the transit of the alluvial materials will be possible to initiate the process of side bars formation.
• Low solid sediment transportation. When the sediment load of the stream is too low, it will be necessary to create artificial structures, similar to the natural alternating lateral bars. It is also important to adapt the structures to resist the flow (even with the use of geotextiles, biodegradable coconut if the berms are vegetated, etc).


Pool-riffle installation: In-channel gravel augmentation can be employed to create pool-riffle series or to modify pre-existing ones. This increases hydraulic and morphological diversity, enhancing river habitat.

Slope creation at regulated rivers for pool-riffle enhancement

Riffle-pool sequences are the dominant bedforms in gravel and mixed bedded channels of intermediate slope. A single riffle cannot be rehabilitated without considering the impact on upstream riffles. When we add gravel at one degraded riffle, the water rises upstream and may flood the next upstream riffle, which can lose its functionality. The backwater effect has to be considered in the design and development of the project, regarding to the maximum high of the gravel bed placed on the glide. One technique to prevent the undesired effects is the staged slope creation. Elkins et al. (2007) in a study at the River Waveney applied gravel augmentation to restore riffle-pool dynamics for habitat enhancement below a dam, so no riffle was affected upstream. The method is indicated for regulated rivers, where longitudinal slope has declined, depth is increased, velocity decreased and substrates become clogged. Always is preferable to reinstate the natural flow regime (see measures to improve flow dynamics: Establish environmental flows / naturalize flow regimes), but this measure is sometimes infeasible.

If the purpose is the recreation of riffles in the middle of a reach, for the maintenance of the riffle-pool sequence the variability of channel width plays an important role. To concentrate the flow through the pools and allow it to dissipate out across a riffle, the width can be constrained upstream of pools (e.g. with wing deflectors) and expanded upstream of riffles.

Applicability

Gravel bar recreation

Recommendations and constraints

• Preliminary study of the geodynamic variables (solid transport, erosion of streambanks and margins) for determining if the restoration would be passive or active, simply adding sediments or accomplished with fully built bars (see adding sediments).
• It is important to simulate the natural bars distribution and dimensions. Former aerial photographs can be used to set a reference state for natural bars at the reach of study, or reproduce those found at unaltered streams of similar characteristics.
• The establishment of side bars can implicate an augmentation of the flood frequency due to the reduction of the channel section. Where the fluvial territory is respected and no hazardous consequences for infrastructures, crops, or recreative places are expected, no defense measures have to be taken. Near urban emplacements it will be necessary to predict the hydrologic effects and check the potential flooding events. The maintenance of the banks and margins is needed to prevent the development of dense vegetation due to the reduction of the flow capacity of the section.


Pool-riffle recreation

Recommendations and constraints

• When designing the gravel bedforms, regard to their potential impact on water elevations (flood levels) and to the flow resistance.
• The benefits of in-channel gravel augmentation may be limited by the maximum riffle crest elevation achievable.
• The “reverse domino” effect, as more riffles crests are rehabilitated downstream, the interplay becomes more complex, and interdependent.
• If the restoration is done in a regulated river, as long as the dam remains, the gravel bedform below the dam has to be maintained with periodical gravel injections.

Expected effect of measure on (including literature citations):

  • HYMO (general and specified per HYMO element)
  • physico � chemical parameters
  • Biota (general and specified per Biological quality elements)

Temporal and spatial response

Pressures that can be addressed by this measure

Cost-efficiency

Case studies where this measure has been applied

Useful references

Other relevant information

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